Allow non-destructive access to the read buffer. (#1600)

I need this to implement SMTP pipelining checks. I mostly need to
flush my send buffer when the read buffer is empty before waiting for
the next command.

tokio-util/src/codec/framed_read.rs

@@ -79,6 +79,11 @@ impl<T, D> FramedRead<T, D> {
     pub fn decoder_mut(&mut self) -> &mut D {
         &mut self.inner.inner.1
     }
+
+    /// Returns a reference to the read buffer.
+    pub fn read_buffer(&self) -> &BytesMut {
+        &self.inner.buffer
+    }
 }
 
 impl<T, D> Stream for FramedRead<T, D>
@@ -174,6 +179,10 @@ impl<T> FramedRead2<T> {
     pub(crate) fn get_mut(&mut self) -> &mut T {
         &mut self.inner
     }
+
+    pub(crate) fn buffer(&self) -> &BytesMut {
+        &self.buffer
+    }
 }
 
 impl<T> Stream for FramedRead2<T>